 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to methods or processes for preparing water
diluted mixtures of acids and, in particular, to the preparation of dilute
sulfuric acid of varying specific gravities for use in the manufacture of
lead-acid storage batteries.
Dilute sulfuric acid is an important component in the manufacturing of
lead-acid storage batteries. It is used to form the paste of the dry
active materials for both the positive and negative electrodes, for the
electrolyte used in the formation process, and for the final battery
electrolyte. As is well known in the art, acids of varying specific
gravities are required for each of the various uses and each particular
use may itself require varying specific gravities, depending on the type
of battery being made. In a typical lead-acid automotive battery
manufacturing plant, sulfuring acid mixes having specific gravities
ranging from about 1.150 to 1.350 are required. Further, precise control
of the specific gravity of each acid mix is critical to the manufacturing
process and, ultimately, to the performance of the batteries.
Many battery manufacturing plants still use simple batch methods for the
preparation of sulfuric acid mixes. In these processes open lead-lined
steel tanks are filled with concentrated sulfuric acid and water or
recycled plant acid while manually controlling the flows based on crude
preliminary calculations utilizing initial specific gravity measurements.
The acid and water are mixed within the tank, the specific gravity is
checked by withdrawing a sample, and acid or water is added by trial and
error until the final desired specific gravity is reached. The exothermic
heat generated during mixing typically raises the temperature of the acid
batch to about 190.degree. to 240.degree. F. (88.degree. to 116.degree.
C.) and continuous recirculation for periods of several hours may be
required to cool the acid to a required temperature of something less than
125.degree. F. (52.degree. C.). The cooled acid is finally transferred to
a storage tank from which it is removed for direct utilization in a
battery manufacturing process. Typically, a series of batch mixing tanks
is required so that several acids of different specific gravities can be
prepared at one time. The entire process is tedious and time consuming.
When acid is batch mixed in open tanks, contamination is unavoidable, acid
fumes are emitted and health and corrosion problems result. In addition,
stratification of the varying densities of acids and water results and a
true measurement of specific gravity is difficult to make. Finally, if
slow cooling by recirculation is employed, corrections must be made for
the resultant variations in specific gravity with reductions in
temperature.
More recently, attempts have been made to automate and provide more direct
control in sulfuric acid mixing processes. It is known, for example, to
provide one-step mixing by combining concentrated acid and dilute acid or
water utilizing simultaneous control over the flow valves from both
component sources. The use of heat exchangers to cool the acid heated
during mixing is also known. Nevertheless, such one-step in-line mixing is
still essentially a batch process and suffers from the same lack of
flexibility. Additionally, it has been found to be extremely difficult to
control the flow of concentrated sulfuric acid in a single stage mixing
process. The amount of concentrated acid typically required to be mixed
with water or lower specific gravity recycle acid to obtain the final
specific gravity acid is relatively small. Consequently, extremely small
adjustments are required in the low volume flow of concentrated acid. With
the valves typically used, the proper adjustments cannot be made or
maintained for the time periods required. The result is poor control of
the final specific gravity and the creation of unacceptably high volumes
of reject acid. Further, it is impossible with prior art one-step mixing
methods to use recycled process acid of a given specific gravity to
produce a desired final acid mixture with a lower specific gravity,
because the single step addition of concentrated acid can only be used to
raise the specific gravity. As a result, battery manufacturing plants may
often generate large volumes of potential recycle acid which, with a
one-step method, can only be used to a limited extent in preparing higher
specific gravity and mixtures. Such plants may thus be faced with
extremely burdensome problems of neutralization or other acid disposal
methods.
SUMMARY OF THE INVENTION
The method of the present invention utilizes two-stage in-line mixing to
provide extremely accurate control of the final specific gravity and broad
flexibility in the range of specific gravity acids required in a typical
battery manufacturing facility. The specific gravities of the mixtures at
each of the two stages are continuously measured and fed to a
microprocessor which compares the measured specific gravities to an
intermediate set point specific gravity and the desired final specific
gravity, respectively, and calculates the deviation in each case. Control
signals are generated from each calculated deviation and utilized to
control one component of flow at each mixing stage. The actual measurement
of flow of any component is obviated.
The desired final specific gravity acid is piped directly to a closed
storage tank. The microprocessor is programmed to readily adjust to make
the calculations and alter the flows to provide acid of any density
required for the battery manufacturing operations. Open mixing tanks and
all of the attendant problems are eliminated. The control problems and
resultant inaccuracies in final specific gravity which are inherent in
one-step in-line mixing are also completely eliminated. The method of the
present invention is adaptable to use recycle and reject acid of virtually
any specific gravity produced in a lead-acid battery manufacturing plant.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE shows a process diagram of an acid mixing system
utilizing the two-stage in-line method of present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the drawing, a process layout shows a typical sulfuric acid mixing
system used in a battery manufacturing plant and embodying the method of
the present invention. The components of the sulfuric acid/water mixtures
of the various specific gravities utilized in the manufacture of batteries
are typically obtained from a number of sources and stored in tanks. For
the first stage mixing, these include a water tank 1 containing ordinary
tap water, a reject acid tank 2 for acid mixed by the present method of an
incorrect specific gravity (as during periods of process adjustment), a
recycle acid tank 3 for process acid used in the battery manufacturing
operation but not consumed, and a concentrated sulfuric acid tank 4 which
supplies concentrated sulfuric acid at its maximum specific gravity of
1.835 to raise the specific gravity of the initial first-stage mixture to
the intermediate set point value, as will be described hereinafter. For
the second-stage mixing and to provide the acid of the desired final
specific gravity, the first-stage mixture is combined with ordinary tap
water from a mixing water tank 5, the flow from which is the controlled
variable in the second stage.
To prepare an acid mixture of a desired final specific gravity, a flow of
water, reject acid, or recycle acid from one of tanks 1, 2, or 3,
respectively, is caused to flow through line 6 by pump 7 at a rate
manually fixed by predetermined calculation. Simultaneously, a flow of
concentrated acid from the storage tank 4 is pumped by pump 8 through line
9 at a flow rate determined by preliminary calculation to provide a
first-stage mixture with a specific gravity of approximately 1.400 (or any
other specific gravity greater than the desired final specific gravity).
This is the target or first-stage set point specific gravity. The flows in
lines 6 and 9 are combined in a mixing T 10. As is well known in the art,
the mixing of concentrated sulfuric acid with water or dilute sulfuric
acid creates an exothermic generation of heat, such that the temperature
of the first mixture in T 10 is heated substantially. The initial
temperature of the first mixture is generally in the range of about
190.degree. to 240.degree. F. (88.degree. to 116.degree. C.). Because high
temperature acid is unsuitable for use in any battery manufacturing
operation, the first mixture is directed from T 10 into and through a heat
exchanger 11. The heat exchanger may, for example, be a carbon block type
using cooling water from a source 12 which is pumped to the heat exchanger
through line 13 and subsequently recirculated to the source 12 via line
14, cooling tower 15, and line 16. Preferably, the heat exchanger will
lower the initial temperature of the first mixture to a range of about
90.degree. to about 115.degree. F. (32.degree. to 46.degree. C.).
It is desirable to maintain the specific gravity of the cooled first
mixture at a set point of 1.400, as mentioned above. The set point
specific gravity is established at a level somewhat higher than the
highest specific gravity of any desired final acid mixture for use in the
various battery manufacturing processes. A specific gravity measuring
device 17 is installed in the outlet line 18 from the heat exchanger 11.
The device may be any one of a number which are commercially available,
for example, a Dynatrol unit manufactured by Automation Products, Inc. The
specific gravity of the first mixture is continuously measured and fed to
a microprocessor 19 which is programmed to calculate any deviation between
the measured specific gravity and the set point specific gravity. The
microprocessor in turn generates a first control signal representative of
the calculated deviation. The first control signal is directed to a
concentrated acid flow control valve 20 in line 9 and operates the valve
to adjust the flow of concentrated acid to establish parity between the
measured specific gravity of the first mixture and the set point specific
gravity.
The first mixture at the set point specific gravity continues to flow
through line 18 to a second mixing T 21 where the second-stage mixing
occurs. In the second mixing T 21, the first mixture is combined with a
second flow of water from storage tank 5 through line 22. The second flow
of water dilutes slightly the first mixture at the set point specific
gravity to provide the desired mixture at the final specific gravity which
exits from the mixing T 21 via line 24. To provide any final required
adjustment to the specific gravity, a second specific gravity measuring
device 25 is installed in line 24 and continuously measured the specific
gravity of the final mixture. The measured specific gravity of the mixture
in line 24 is fed to the microprocessor 19 where it is compared with the
stored value of the preselected desired final specific gravity. The
microprocessor automatically calculates any deviation between the measured
specific gravity and the desired final specific gravity and generates a
second control signal which, in turn, is used to adjust a water flow
control valve 23 in line 22. The second flow of water through valve 23 is
automatically adjusted to establish parity between the measured specific
gravity in line 24 and the desired final specific gravity.
The mixing method employed by the foregoing process is readily adaptable to
provide a continuous flow of acids at various desired final specific
gravities. During the period of initial adjustment of the system or when
changing from one final specific gravity acid to another, the continuous
flow of acid at an incorrect specific gravity in line 24 is diverted to
the reject acid tank 2 by opening valve 27 and directing the flow through
the reject acid line 26. This intermediate diversion is also automatically
controlled by applying the second control signal generated by the
microprocessor. When the two-stage adjustments have been properly made to
provide a final desired specific gravity flow in line 24, valve 27 is
automatically closed on a signal from the microprocessor 19 and the
appropriate one of the valves 28 through 32 controlling the flow to final
acid storage tanks 33 to 37, respectively, is automatically opened by the
control signal from the microprocessor and the flow of acid at a desired
final specific gravity is directed from line 24 through line 38 to the
appropriate final acid storage tank. When, for example, the final acid
storage tank 33 is filled, a level sensing device in the tank operates to
close the fill valve 28. Simultaneously, that valve closure signals the
microprocessor to reopen valve 27 to the reject acid tank 2 and to
generate a new second control signal to adjust the second water flow
control valve 23 to provide the next preselected final specific gravity
acid flow in line 24. Until the flow in line 24 is properly adjusted for
the next desired final specific gravity, the mixed acid flow in line 24 is
diverted via line 26 to the reject acid tank 2. The eventual establishment
of parity between the actual specific gravity as measured by the unit 25
in line 24 and the next preselected desired final specific gravity results
in the generation of appropriate control signals to close valve 27 and
open, for example, valve 29 to final acid storage tank 34 for receipt of
the acid at the next desired final specific gravity via line 38. The
process continues to automatically repeat and provide varying final
specific gravity acids for storage tanks 35, 36 and 37, as may be
required.
The microprocessor 19 used to provide control for the method of the present
invention may be, for example, a Modicon model 484 programmable controller
having analog input and output capability. The microprocessor is
programmed to apply digital PID (proportional integral derivative) control
algorithms to produce output control signals to conventional pneumatic
actuators for adjusting or operating the various valves in the system. The
specific gravity of the mixture measured by either of the specific gravity
measuring devices 17 or 25 is converted to an output voltage signal
varying from 1 to 5 vdc to the microprocessor 19 where the deviation from
the set point or the final specific gravity is calculated and an output
control signal varying from 4 to 20 milliamps is generated. The output
signal is converted in a current to pressure transducer to a pneumatic
signal which operates an actuator for adjusting the concentrated acid flow
control valve 20 or the water flow control valve 23.
It is important to note that the method of the present invention does not
require the actual measurement of the volume of flow of the acid mixtures
or any of their components at any point. As a result, flow meters are not
required. The microprocessor memory is also utilized to store the control
signals previously generated and used for a particular final specific
gravity acid. This information may be subsequently recalled from the
microprocessor memory and used to reduce the time for process
readjustment. As a result, a final specific gravity of any desired mixture
within the range typically required can be achieved in less than seven
minutes, and, in most cases, in less than four minutes. Overall, the
process time is kept to a minimum and the generation of undesirable
volumes of reject acid is also minimized. A set of alarm conditions, such
as excessively high temperature acid from the heat exchanger 11 or a lack
of component flow from any one of the mixing sources, may also be
monitored by the microprocessor and used to register an alarm and/or shut
down the system in the event a problem occurs.
It is also possible to establish the set point specific gravity of the
first stage mixture by adjustment of the water or dilute acid component of
the mixture, rather than the flow of concentrated acid. In that case, the
flow of concentrated acid would be preset and maintained at a constant
rate, and an appropriate water, reject acid, or recycle acid flow control
valve 41, 42 or 43 from tank 1, 2, or 3, respectively, would be operated
by the first control signal from the microprocessor in essentially the
manner previously described. Such control, however, would limit somewhat
the flexibility of the process to provide acid mixture with the desirable
wide range of final specific gravities.
The specific gravities of battery manufacturing acids are typically
measured to three decimal places with an allowable tolerance of .+-.0.003.
Control of final specific gravities well within this range is easily
attained in the process described herein. A typical system, operating as
described, can produce a continuous flow of varying specific gravity acids
of from 25 to 35 gpm which is sufficient to provide the needs for a
battery plant manufacturing up to 15,000 batteries per day.
* * * * *
|
|
 |
|
 |
Description |
|
